1. Field of the Invention
The present invention relates to an overhead cable designed to reduce the wind load.
2. Description of the Related Art
As an overhead cable of the related art, much use is made of a steel-reinforced aluminum cable (ACSR) comprised of aluminum strands twisted together along the longitudinal direction around a steel core. As such an overhead cable designed to reduce the wind load of the overhead cable, as shown in FIG. 1, a cable obtaining by twisting together aluminum strands 2 on a steel core 1, twisting together segment strands 3 along the longitudinal direction having a sector-form cross-section at the outermost layer, forming the corners of each segment strands 3 as arc surfaces, and preventing the tangent of the arc of the intersection of the adjoining abutting surfaces of the segment strands 3 and the arc of the corners from passing through the center of the cable by setting the radius of curvature of the corner arc surface to a specific value and thereby reduce the wind load is disclosed in Japanese Examined Patent Publication (Kokoku) No. 57-46166.
As another overhead cable designed to reduce the wind load, an overhead cable obtained by setting the height of projections of spiral strands wound over an envelope of the outermost layer strands and the center angles of the projections to specific values so as to reduce the wind load is disclosed in Japanese Examined Patent Publication (Kokoku) No. 5-6764.
As still another overhead cable designed to reduce the wind load, an overhead cable obtained by making the surface of the outermost layer a wavy shape so as to reduce the wind load is disclosed in Japanese Examined Patent Publication (Kokoku) No. 7-34328.
These overhead cables above mentioned can give effect of reducing the wind load.
Other type of overhead cable designed to reduce the wind load is disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-50814.
The overhead cable disclosed in the publication, as shown in FIG. 2, is an overhead cable obtained by twisting together aluminum strands 2 on a steel core 1 along the longitudinal direction, twisting together segment strands 3 having a sector-form cross-section at the outermost layer and providing arc-shaped grooves 4 at the front surface of the adjoining parts of the segment strands.
This overhead cable has a circular sectional shape as its basic shape. It is formed with arc-shaped grooves in its front surface. The swirl created inside the grooves causes the breakaway point of the flow to move to the downwind side of the overhead cable and thereby reduces a drag coefficient Cd.
The overhead cable disclosed in Japanese Unexamined Patent Publication (Kokai) No. 8-50814 gave the effect of reducing the wind load when the diameter of the overhead cable was relatively large, but had the disadvantage that when the diameter of the overhead cable became small, the Reynolds number Re fell and, due to the relation of this lowered Reynolds number Re and the drag coefficient Cd, the wind speed zone in which the drag coefficient Cd was lowered became remarkably high. For this reason, in an overhead cable having a small diameter, for example, a diameter of 25 mm or less, the design wind speed at which the drag coefficient Cd fell became 60 to 70 m/s, which was not practical.
An object of the present invention is to provide an overhead cable designed to reduce the wind load acting upon the overhead cable in a lower wind speed zone even in a cable having a relatively small diameter.
According to the present invention, there is provided an overhead cable comprising: an outermost layer constituted by twisting together a plurality of segment strands, and having a spiral groove along the longitudinal direction in the outer circumferential surface region of each boundary portion of adjoining segment strands, wherein in the contour of the cross-section of said outermost layer, each groove comprises an arc-shaped curve having a predetermined radius R centered about a vertex of a regular polygon and each part between adjoining grooves comprises a straight line or a curve which is concave with respect to said straight line.
In the overhead cable of the present invention, the fluctuation in pressure created at the cable surface depends upon the shape of the polygon and the fluctuations in pressure are made to occur at the vertexes of the polygon. As a result, the distribution of speed inside a laminar flow boundary layer collapsed to cause turbulence at an early stage and therefore increase the speed in the bottom of the boundary layer.
For this reason, the breakaway point of the flow moves to the back flow side, a back flow zone of the cable is reduced, a negative pressure area generated downwind of the cable is reduced, and therefore the drag becomes smaller.